Tunneling Magnetoresistance (TMR) Switches are polarity sensitive devices that respond to a magnetic field.  A correctly oriented magnet will turn them on and off.

A reed switch consists of two ferrous leads sealed inside a glass tube.  With an appropriate magnetic field applied, the leads will attract each other and snap together.

A Giant Magnetoresistive (GMR) switch turns on when a magnet is brought near.  It is not polarity sensitive.  When a magnet in the correct orientation approaches, the GMR switch will activate.

Bipolar switches are similar to latches.  They activate with one pole and then deactivate with the opposite pole.   Unlike a latch, the response to south and north pole is probably not symmetric.  A particular device might respond more to one pole than the other.

An omnipolar switch is like a unipolar switch except that it responds to either magnet pole. As either pole of a magnet nears the switch, it will activate.

A Hall-effect latch is a device that is activated by one pole of a magnet and deactivated by the other.  In many types of latches, the south pole activate the latch and the north pole deactivates.  Once the latch has been activated or deactivated, it stays in the same electrical state until the other magnet pole is brought near.

Why do magnets attract some objects?  Some objects can be magnetized.  If a magnet comes close, the object turns into a temporary magnet.  It is then attracted to the magnet.

Two important magnet material properties are strength and resistance to demagnetization.  A BH curve shows how these are related.

A magnet's shape affects how much self-demagnetization occurs inside a particular magnet.  This affects the external field produced by the magnet.

Simulating fields from bar magnets used for sensor applications is among the easiest electromagnetic simulation tasks.  There are two main methods for doing this:  finite-element analysis (FEA) software, and direct solutions of Maxwell's equations.

Permanent magnets can come in a variety of shapes and magnetizations.  Many sensing applications need only a simple bar magnet.  This article talks about bar magnets.

All of magnetism can be boiled down to a few simple ideas.  If you understand these, you will be in a position to understand most magnet and sensor concepts.